This invention relates to galliobismuthate glasses with superior infrared transmitting ability and a high nonlinear susceptibility, thus making them suitable for various optical devices.
Active optical devices, which are currently being studied for use in future telecommunications and computational systems, require optically nonlinear materials. In such materials a light beam will change the optical properties of the medium, altering the propagation of other light beams, called cross-modulation, or of itself, called self-modulation. Many of the proposed active optical devices also require that the material have a fast response time and low optical loss. Only certain glasses satisfy these requirements.
Such a class of glasses has been identified by several workers in their investigations of heavy metal oxide (HMO) glasses, wherein the superior infrared transmitting ability of those glasses was discovered. Those glasses are unusual in that they contain none of the traditional glass-forming oxides, such as those of silicon, boron, germanium, and phosphorous, yet they exhibit remarkable stability. Many such glasses have been previously patented, but principally for infrared transmitting applications. Subsequent investigations led to the discovery that these glasses also display a high degree of nonlinear susceptibility. Measurements on representative HMO glasses have indicated a nonlinear refractive index some 50x that of vitreous silica. This factor makes these materials suitable for a range of optical applications, including areas where optical bistability, beam steering, optical phase conjugation, and bistability of resonator structures are desired. These applications require that the optical properties of the material change in response to an applied optical field, the size of this effect being characterized by a single term called the third order susceptibility tensor, X.sup.(3), which can be thought of as a coefficient in a power series expansion of the relationship between the applied electric field, E, and the polarization, P, written schematically as EQU P=X.sup.(1) E+X.sup.(2) EE+X.sup.(3) EEE+. . . ,
where X.sup.(1) is the linear susceptibility tensor, X.sup.(2) is the second order susceptibility tensor, and so on.
One example of the physical changes that may occur in a glass is the change in the index of refraction brought about by the presence of a high-intensity light beam. When a light beam of intensity I is passed through a nonlinear medium, the index of refraction can be described by the equation EQU n=n.sub.o +.gamma.I,
where n is the index of refraction, n.sub.o is the linear (low intensity) index, and .gamma. is the nonlinear refractive index. The nonlinear refractive index, in turn, is related to the susceptibility tensor by the equation ##EQU1## where c is the velocity of light.
To effectively utilize these material properties in optical applications such as low loss waveguides and photonic switches, it is necessary to produce optical fibers based on these glass compositions. Consequently, a need was identified to make HMO glasses in fiber form. Both the core glass and the cladding glass must be transparent in the infrared region when optical fibers are used in the infrared region, which suggests that both the core and cladding glass compositions should be selected from glasses of generally similar compositions.
The preferred architecture of these optical waveguides is one which confines the light tightly to the core region of the fiber. A typical architecture employed in a single mode type optical fiber is the step index profile, which consists of a relatively small core region surrounded by a relatively large cladding with a lower refractive index. Thus, construction of a glass fiber with the appropriate properties to become an effective optical waveguide requires selection of two different glass compositions for the core and cladding having a compatibility of material properties and processing viscosities.
An effective optical waveguide fiber requires a difference in refractive index between the core and cladding. This difference is usually described in terms of the relative refractive index, .DELTA., defined by the equation EQU .DELTA.=[n.sub.1.sup.2 -n.sub.2.sup.2)/(2n.sub.1.sup.2),
where n.sub.1 is the refractive index of the core glass and n.sub.2 is the refractive index of the cladding glass. A .DELTA. of .apprxeq.0.35% is typical in a standard optical waveguide fiber, whereas, a .DELTA. of .apprxeq.2.5% is commonly featured in the waveguides described in the instant invention.
While smaller .DELTA. values would allow the fabrication of a functional waveguide, these high .DELTA. values were chosen for two reasons. First, because the fibers described in the instant invention are fabricated from melted glass instead of via the tightly controlled vapor deposition process conventionally used to form waveguides, the refractive index thereof cannot be as easily controlled. Thus, a larger .DELTA. value allows the tolerance of more variability in the relative refractive index while still achieving a functional waveguide. Second, the nonlinear effects of interest are all a function of light intensity, which is power per unit area. When a given amount of optical power emanates from a source, such as a laser, and into an optical waveguide fiber, light is more tightly confined to a smaller core region. This tighter confinement is made possible by higher .DELTA. values and results in larger nonlinear effects.
The material compatibility requirements of an effective optical waveguide fiber necessitate a similarity between the thermal expansion coefficients of the core and the cladding, with the core desirably having a slightly higher thermal expansion. Likewise, the processing compatibility requirements of such an optical fiber necessitate a similarity between the processing viscosities of the core and cladding, with neither glass being prone to devitrification at the processing temperatures.
Accordingly, the primary object of the present invention was to define a series of HMO glass compositions exhibiting high nonlinear susceptibility and superior infrared transmitting ability. Another object of the instant invention was to identify novel HMO glass compositions that are suitable for making optical waveguide fiber. Yet another object of the present invention was to devise effective methods of processing these HMO glasses without allowing crystallization of these glasses. A still further object of the instant invention was to provide a method of producing HMO glass optical waveguide fibers with similar processing viscosities between the core and cladding glasses, and without crystallization of either glass.
The enhanced nonlinearity of the HMO glasses described herein and waveguide structures that may be synthesized from these glasses make them useful in a number of device configurations, some of which can only be implemented in waveguide form (either fiber or planar waveguides), and some of which can also be implemented in bulk-optics form. Such devices include, but are not limited to: nonlinear mode coupling devices, nonlinear interference devices, and optical amplifiers, and areas where optical phase conjugation is desirable.
Nonlinear mode coupling devices operate through changing the coupling of two (or more) modes of a waveguide structure as a result of the third order susceptibility. They include various multi-core couplers and single-core devices where two or more modes of the waveguide structure (such as modes with different polarizations or spatial distributions) have their coupling altered through nonlinear interaction.
In nonlinear interference devices the relative phases of two or more light beams (or even various reflections of a single light beam) are changed by utilizing variations in the optical path length resulting from the third order susceptibility. Such differences are brought about by the change of index of refraction due to the nonlinearity. Representative of this group of nonlinear interference devices is a bulk or guided wave Mach-Zehnder interferometer, although Sagnac interferometers, Michelson-type interferometers, distributed feedback grating devices, and Fabry-Perot resonators may also be included.
When these HMO glasses are used in synthesizing optical amplifiers, the gain coefficients for stimulated Raman and Brillouin amplification are also enhanced. This gain can be used to amplify a signal beam using a pump beam in a guided-wave geometry.
In areas where optical phase conjugation is desired, four-wave mixing interactions (bulk or guided wave) are utilized, wherein three input optical waves interact via the third order susceptibility to form a fourth wave, called a phase conjugate wave, which has unique properties. These properties can be exploited for such uses as aberration corrections, optical memory, beam steering, generation of new wavelengths, and neural networks.
The utility of the instant invention is manifested in the aforementioned general areas of application as well as in devices in which these concepts are used. The inventive HMO glass compositions that make possible the construction of such devices can be summarized as follows.